Illegal immigrants, who were rescued by the Libyan coastguard in the Mediterranean off the Libyan coast, arrive at a naval base in the capital Tripoli on May 26, 2017. At least 20 boats carrying thousands of migrants on their way to Italy were spotted off the coast of the western city of Sabratha, the Libyan navy said. MAHMUD TURKIA / AFP

The revelation that migrants are being sold as slaves in Libya is not an isolated case: more than 40 million people were estimated to be victims of modern slavery in 2016.

The figure includes some 25 million people who were in forced labour — made to work under threat or coercion — and 15 million people in forced marriage, according to a report.

But the number of people in slavery is probably even higher, according to the International Labour Organization (ILO), the International Organization for Migration (IOM) and human rights group Walk Free Foundation, who jointly carried out the study.

Here are some of their key findings:

Forced labour
Around a quarter of the nearly 25 million people in forced labour are in private households but they can also be found in factories, on construction sites, on farms and fishing boats.

The study gives the example of 600 fishermen held on boats in Indonesian waters for many years.

More than half of the slaves are in debt to whomever is holding them. Others cannot escape because they have been drugged, paid too little to be able to leave, physically abused or simply because they are too far from home to make the return journey.

Nearly five million among them are forced into prostitution, the report said. Just over four million are victims of work imposed by state authorities such as forced labour in prison or abuse of army conscripts.

Women and girls
Women and girls account for 71 percent of the slaves, or nearly 29 million people.

One in four victims of modern slavery is a child — around 10 million.

Around 15.4 million people are married against their will, of whom more than a third are below 18 years of age. Practically all are female.